Comparison of ornithopter wind tunnel force measurements with free flight

Developing models of flapping-winged fliers in free flight is vital for accurate control. The aerodynamics associated with flapping-winged flight are complex. Hence, a look-up table flight force model from wind tunnel data is a practical approach. In this work, we compare the flight of an ornithopter micro aerial vehicle (MAV), using free flight data collected from a Vicon motion capture system, to measured wind tunnel force and moment values. We compare the two data sets at equilibrium as a metric to determine the quality of the wind tunnel flight force estimation. To compare the two data sets, we find the predicted equilibrium angle of attack and velocity for the ornithopter in free flight. For a given flapping frequency and pitch control elevator deflection angle at free flight equilibrium, we compute the level sets at zero for pitch moment, net horizontal force, and net vertical force from the wind tunnel data. We then use the point on the zero moment level set that minimizes the vertical and horizontal force. The angle of attack and velocity at this minimal point are the wind tunnel predicted equilibrium point and are compared to the analogous free flight equilibrium point. We determined that the wind tunnel underestimates the angle of attack of the equilibrium point observed in free flight by 15 degrees, while the equilibrium velocity has an error of 0.1 m/s between the two sets at an average flight speed of 2 m/s.